Polyaluminum chlorides and polyaluminum chlorosulfates are compounds used as flocculents and coagulants for water and wastewater treatment. Compared with other inorganic purification agents, these compounds generally work more efficiently, produce less by-product sludge, work better in cold water, settle faster and reduce the pH of water to a lesser extent. The compounds are also used in the production of paper, antiperspirants, foods and pharmaceuticals.
A number of processes have been designed for producing polyaluminum chlorides and polyaluminum chlorosulfates with characteristics favorable for water treatment. For example, U.S. Pat. No. 5,246,686 discloses a process in which basic aluminum chlorosulfate is reacted with an alkaline-earth metal compound (e.g., calcium carbonate) to produce high basicity polyaluminum chlorosulfates. One drawback of this reaction is that it results in the formation of insoluble alkaline-earth sulfates (e.g., gypsum) which must be removed from product solutions by processes such as filtration or centrifugation. This makes the reaction undesirable in high volume industrial operations. Similar problems involving the formation of precipitates have been encountered in processes involving the reaction of aluminum sulfate with a slurry of calcium carbonate and calcium chloride (see e.g., U.S. Pat. No. 4,981,673 or U.S. Pat. No. 5,076,940).
A process which avoids the formation of gypsum-type precipitates is described in U.S. Pat. No. 5,348,721. The disclosed method requires the initial production of a polyaluminum chloro-sulfate of relatively high basicity (40-50%). This is then reacted with an alkaline-earth metal compound (e.g., calcium carbonate) and an alkaline-metal compound (sodium carbonate) at a temperature of 50 to 70 degrees centigrade. Reactions performed at lower temperatures (40 degrees centigrade) result in a product that coalesces into a gel. One problem with this process is that the generation of CO.sub.2 may cause processing problems when reactions are performed on a large scale. Nevertheless, it appears that this is the only reported process which is capable of making polyaluminum chlorosulfates of greater than 67% basicity and a preparation which is 75.3% basic is disclosed in the patent specification. Because of the chemistry involved, all products are produced using substantial amounts of calcium carbonate.
An alternative process is disclosed in U.S. Pat. No. 3,929,666. In this patent, a solution containing sulfate ions is mixed with a solution containing both aluminum ions and halide ions and with a solution containing sodium or potassium aluminate. The reaction is carried out at a temperature of 40 degrees centigrade or less and produces a gel which must then be dissolved by raising its temperature to between 50 and 80 degrees centigrade. Because of the difficulty in efficiently pumping gels from one location to another and because of the problems which gels present in terms of mixing and heat transfer, the process disclosed in this patent is of limited value in large scale industrial applications.
The present invention is directed to a process which avoids many of the problems associated with previously disclosed methods. It can be used to produce compounds with basicities of greater than 70% using, as starting material, basic aluminum chlorides and basic aluminum chlorosulfates of low basicity (25% or less). The polyaluminum chlorides and polyaluminum chlorosulfates produced remain fluid, i.e. they do not coalesce into a gel, and the formation of calcium sulfate precipitates can be avoided. Further, they can be produced without the addition of heat, thus avoiding the formation of undesirable byproducts. As a result, the process is particularly well-suited to large scale industrial applications.
In addition, the process permits the formation of products with unique properties that are also part of the invention. Polyaluminum chlorosulfates can be produced which are greater than 75.3% basic and preparations of greater than 70% basicity can be produced without substantial amounts of calcium carbonate. In addition, concentrated preparations of high basicity polyaluminum chlorides and polyaluminum chlorosulfates can be obtained which contain Al.sub.13, the form of aluminum generally believed to be the most efficient at removing impurities from water.